A pressurized water Nuclear reactor (PWNR) has a core immersed in water in a large steel tank. The fuel rods and control rods make up a vertical array. The control rods are movable, and are pulled up above the fuel rods when the plant is in full operation. The purpose of the control rods is to absorb neutrons which trigger the splitting of atomic nuclei in the fissionable material in the fuel rods. With all the control rods inserted, there is negligible fission (and heating) in the fuel rods. When the control rods are pulled out the fuel rods heat the water, which is circulated by pumps in the primary or, inner loop, to a heat exchanger.
A feature of this design is that only the water in the inner loop is in contact with the radioactive fuel rods. Thus, only the inner loop has contamination from the inevitable small amount of rust and corrosion. There are filters in this inner loop to capture the small particles which are radioactively contaminated. There are additional pumps to circulate cooling water through the core, which form the Emergency Core Cooling System (ECCS). It is essential that circulation be maintained to carry heat away from the fuel rods to prevent them from melting in the event that the main primary circulation pumps should fail. The water in the tank and the primary circulation loop is never supposed to boil, and thus always remain as water because steam is a much poorer conductor of heat as compared to water. The fuel rods are supposed to always stay under water. To prevent boiling, the tank and primary loop are maintained at very high pressure.
The secondary loop water is heated through a heat exchanger with the primary loop. Water in the secondary loop is allowed to boil in a steam generator tank. The steam is used to drive a turbine which turns an electrical generator. The residual steam is condensed back to water, which is pumped back through the heat exchanger again to make more steam. Also, circulation usually through a large cooling tower which is used to remove the waste heat.
One problem with conventional thermal transfer fluids used in PWNRs is the onset of a heat transfer condition that can lead to a departure from nuclei boiling (DNBR) that occurs at a condition call the critical heat flux. (CHF) which results in a blanketing of the fuel rod with bubbles that interferes with heat transfer and leads to a condition called dryout that can result in a critical failure of the fuel rods. What is needed is nanofluid having enhanced thermal transfer and stability for PWNRs to raise the heat flux level at which dryout condition will occur. This heat flux level is called the critical heat flux (CHF).